Electrodeless apparatus for the formation and containment of a plasma



y 4, 1967 A. MICHEL ETAL 3,329,864

ELECTRODELESS APPARATUS FOR THE FORMATION AND CONTAINMENT OF A PLASMA Filed Oct. 19, 1965 2 Sheets-Sheet 1 Fug y 4, 1967 A. MICHEL ETAL 3,329,864

- ELECTRODELESS APPARATUS FOR THE FORMATION AND CONTAINMENT OF A PLASMA Filed Oct. 19, 1965 2 Sheets-Sheet 2 2 STAGE m st/ass:

United States Patent Claims. Ci. 315-111) Our invention relates to method and apparatus for producing or accelerating plasmoids without the aid of electrodes by means of mutually intersecting electrical and magnetic fields. Such method and plasma apparatus for carrying out a method of this kind are described and illustrated in the copending application of A. Koller, A. Michel and H. Schindler, Ser. No. 349,996 filed Mar. 6, 1964.

Apparatus according to the copending application comprise an insulating tubular housing on which are mounted two coaxial field coils of mutually opposed winding sense, which are axially spaced from each other for jointly producing a cusp field, a coaxially wound induction coil being disposed between the two field coils. The magnetic field is produced by the two field coils which are traversed by currentin such a manner that the magnetic field between the two coils has a radial component (cusp field). The electrical field is induced by a rapidly varying magnetic field of the intermediate induction coil which is pulsewise in operation. According to the copending application, the cusp field in the space between the field coils is maintained while an ionized gas (plasma) is introduced into the plane of the induction coil. As the plasma arrives in the plane of the induction coil, this coil is pulsewise excited electrically, and the thus resulting magnetic field induces a circular electrical field which causes a plasma ray current to flow within the cusp field. The coaction of the electrical and magnetic fields causes the occurence of a Lorentz force which ejects the plasma in the direction of the tube axis out of the plane of the induction coil.

Referring to plasma apparatus, and the method carried out thereby, generally of the kind described in the foregoing, it is an object of the present invention to afford imparting to the entire mass of a plasmoid in an electrodeless plasma accelerator a subsequent acceleration and/ or reflection while simultaneously heating the plasma additionally to a higher temperature.

Another object of the invention is to provide an improved plasma ejection device so that the jet of plasma issuing therefrom becomes utilizable outside of the plasma vessel or may be used for such purposes as the propulsion of missiles or satellites.

To achieve these objects, and in accordance with the invention, we provide for after-acceleration and/or reflection of plasmoids in an electrodeless plasma accelaration tube on which are mounted two or more field coils coaxially beside one another and in mutually spaced relation and alternately opposed winding sense so as to produce intermediate cusp fields. Furthermore we provide induction coils, likewise of alternately opposed winding sense, which are coaxially located between the field coils. According to an essential feature of our invention, the plasma, prior to being emitted from the second or further cusp-field stage, is collected during a collecting phase lasting a few microseconds and is simultaneously heated to higher temperature; and thereafter the induction coil appertaining to the particular cusp-field stage is pulsewise electrically excited. The mentioned collecting phase may last about five microseconds, for example.

The improvement afforded by the present invention is to be seen primarily in the fact that the propelled plasma is caught in the magnetic cusp fields and consequently is 3,329,864 Patented July 4, 1967 stored in these fields, whereby the oriented energy of the plasma is largely thermalized, which causes the electrical conductivity of the plasma to increase. Consequently, with respect to the after-acceleration, there is available a larger total mass of higher initial conductivity than with the method and apparatus according to the copending application mentioned hereinabove.

According to another feature of the present invention, the plasma is reflected back and forth several times between two cusp-field stages; and prior to each reflection the plasma is collected by one of the cusp-field stages during the collecting phase lasting a few microseconds each time, thus being step-wise heated to higher temperature while being dynamically confined.

. For further explaining the invention, the after-acceleration as well as the reflection of plasmoids after collection and heating within a magnetic cusp-field will be described with reference to a two-stage plasma accelrator schematically shown on the accompanying drawings in which:

FIG. 1 shows schematically and by way of example a plasma accelerator with two cusp-field stages, and

FIG. 2 is an explanatory graph relating to the plasma travel in the accelerator according to FIG. 1.

The apparatus illustrated in FIG. 1 comprises an insulating tubular vessel 7 which carries coaxial coils 1, 3 and 5 alternately wound in opposed sense to produce quasi-stationary magnetic fields 8, which at a sufficiently large distance from the coil ends are axially directed and homogeneous. The magnetic fields 8, therefore, exhibit a cusp geometry between the coils 1 and 3 as 'well as between the coils 3 and 5. Located between coils 1 and 3 is a low-induction coil 2, another low-induction coil 4 being coaxially situated between the field coils '3 and 5.

Before start-up of the plasma accelerator, the insulating tube 7 is evacuated by means of a vacuum pump 15 down to a negative pressure of about 3.10- torr. During operation of the equipment, a valve 16 is opened. This is effected, for example, by discharging a capacitor 37 to actuate the valve 16, for example, a solenoid valve. Within a short interval of time the valve 16 thus permits an acunately metered quantity of gas to flow from a storage container 18 at the left end of tube 7 into the tube. As soon as the front of this acurately metered gas quantity reaches the coil 6, a high-frequency electric current is passed through the coil 6, for example, by oscillatory discharge of a substantially induction-free capacitor 38.

As a result there is produced a high-frequency electrical field in the interior of the tube 7, causing a preionization of the gas.

A few microseconds after commencement of the preionization, when the preheated plasma has arrived in the first cusp stage between coils 1 and 3, a rapidly increasing current is passed through the induction coil 2. The variable magnetic field of this current induces in the interior of the insulating tube 7 a circular electrical field 10. The induction coil 2 is preferably excited by discharging through the coil 2 an induction-poor battery 39 of capacitors. The circular electrical field 10 produces in the plasma an azimuthal flow of current relative to the cylindrical geometry of the insulating tube 7. This flow of current coacts with the magnetic field 8 in such a manner that it produces, together with a radial component of the magnetic field identified by the arrows 11, a Lorentz force in the direction of the tube axis and also produces, together with the axial component of the magnetic field, a Lorentz force in the radial direction, namely toward the tube axis.

The consequence in a compressing axial acceleration of the plasma in the direction indicated by the arrow 9. The ejected plasmoid ultimately moves in the homogeneous magnetic field of the coil 3 which acts as a guiding field and impedes any diffusion of the plasma toward the wall of the tubular vessel 7. Within thisguiding field the plasma 18.8 the shape of a cylinder of approximately 10 to 20 :m. length and 3 cm. diameter. When the plasma cylinder s subjected to after acceleration at the moment when it arrives in the second cusp stage, namely, in the plane of :he coil 4, only a portion of the plasma can be subjected such acceleration because the plasmoid is longer than :he acceleration center. In the drawing, the coil 4 has a poling different from that of the coil 2.

According to the invention, therefore, the moment when the after-acceleration commences is delayed by a few microseconds, for example 5 microseconds, in order to ielay the plasma in the cusp-field, to confine it for some .nterval of time and to thus store the entire amount of plasma. During this interval, a portion of the kinetic energy of the plasma is converted to thermal energy. The resulting increase in plasma temperature simultaneously causes an increased electrical conductivity of the plasma. Hence it is possible to induce in the second cusp-field stage higher current densities than can be effected with the equipment and method heretofore proposed in the aforementioned copending application. An increase in the Lorentz forces is therefore obtained and thus also a greater acceleration of the plasma. When a rapidly increasing current is passed through the coil 4, an azimuthal electrical field 12 will result. This produces, together with the radially inwardly directed magnetic field 13 in the second cusp-field stage, an acceleration of the plasma in the direction indicated by the arrow 14, for example toward a plasma ejection device 17. The plasma is accelerated in opposition to the direction of arrow 14, namely toward the coil 1 i.e. in a backward direction, if the coil 4 is poled in the same manner as the coil 1.

The performance of apparatus according to the invention, as exemplified in FIG. 1, will now be described in greater detail.

After evacuating the discharge tube 7 by means of the pump 15, the spark gap 31 is triggered by the ignition device 30 in order to discharge the capacitor 37 and thereby open the valve 16 for a short interval of time. A Rogowski-belt 36 inductively placed about a lead 43 of the spark gap 31 acts as a transformer which initiates the operation of the time delay stage 42. A Rogowski-belt, in principle, is a coil placed about the current-conducting lead 43. The time delay stage 42 then ignites the spark gap 35 which discharges the capacitor battery 41 in order to excite by the capacitive energy content the coils 1, 3 and 5 through a series-connected resistor 44. Now the above-described cusp fields between the field coils are being built up approximately in the planes of respective coils 2 and 4.

Thereafter, the time delay stage 42 acts to ignite the spark gap 32 at the moment at which the front of the gas coming from the valve 16 arrives at the plane of the coil 6. After the ignition of the spark gap 32, the capacitor 38 is discharged through the coil 6 in order to ionize the arriving gas, i.e. to convert it to plasma. If the arriving gas has previously been ionized sufficiently, the coil 6 with the appertaining circuit may be omitted.

When the plasma arrives in the plane of the coil 2, the spark gap 33 is ignited likewise by means of the time delay stage 42. The capacitor 39 is then discharged through the spark gap 33 so that a rapidly increasing current flows through the coil 2 and, as described above, compresses the plasma and ejects it with acceleration out of the plane of coil 2 in the direction toward the coil 4.

According to the invention, however, the coil 4 is not ignited immediately whenthe front of the plasma arrives in the plane of the coil 4. Ignition of the coil 4 rather takes place a few microseconds later, after the plasma has collected in the cusp field of the coils 3 and 5. According to the invention, therefore, the interval of time chosen to expire between the arrival of the plasma front in the plane of coil 4 and the ignition of the coil 4 is such that the above-described storing and heating of the plasma in Lil the space of the coil 4 can take place. For example, 5 microseconds may be provided as storage interval. After this interval has elapsed, the time delay stage 42 ignites the spark gap 34 and the capacitor 40 is discharged through the coil 4. Now the plasma is again compressed, and, depending upon the poling of the coil 4, is accelerated in one or the other direction and thus ejected out of the plane of coil 4.

In an embodiment constructed and operated according to the invention, the capacitors 37 to 41, in the sequence given, have the following capacitance values: 4.6,uf., .3,Ltf., 15,u.f., IS/Lf. and M. The capacitors 37 and 39 to 41 are subjected to a voltage of 15 kv., and a voltage of 20 kv. is applied to the capacitor 38. The seriesconnected resistor 44 has a resistance of 10 ohms.

FIG. 2 shows a time diagram of the plasma travel in the above-described plasma accelerator according to FIG. 1. Plotted along the abscissa is the time from the beginning of a discharge in the first cusp-field stage in microseconds. The ordinate corresponds to the tube axis of the plasma accelerator. In the locus-time diagram, according to FIG. 2, a plasma first travels along the line 21 from the first to the second cusp-field stage where it remains temporarily at rest according to line 22. After a pause-in the present example of about 5 microsecondsthe second cusp stage is ignited and the plasma is again emitted, this time in the direction 23. The velocity of the plasma has increased by a factor of 5 to 6.

In the embodiment represented by FIG. 1, the discharge through the coil 4 constitutes a damped oscillation. For that reason plasma is again ejected out of the second stage after a lapse of an interval of about 5 microseconds. This time, however, a reflection in the direction 24 takes place, namely from the second stage in the direction toward the first stage, due to the fact that the polarity of the discharge through the coil 4 has become reversed in comparison with the first plasma ejection 23. If a further amount of plasma is located in the second stage, then repeated plasma ejections occur in the directions 25 and 26, each succeeding plasma ejection having a lower velocity than the preceding plasma ejection. This is due to the fact that the effective Lorentz force at each plasma ejection is smaller than during the preceding rejection, because of the attenuated oscillatory discharge through the coil 4.

By reversing the discharging current flowing through the coil 4, the reflection 24 can be made to occur during the first half-wave. In this case the plasma located in the second stage is shot back to the first cusp-field stage by the first plasma ejection 23.

The illustrated mechanism is not limited to the particular embodiment shown and described herein, but can be modified in various respects. For example, the mechanism can be repeated simply by adding further stages of the same design and performance. It is also possible to heat the plasma stepwise to higher temperatures while confining it dynamically by repeated reflection between two accelerating stages. In this manner a plasma may be produced which possesses an accurately prescribed and almost continuously adjustable energy.

Such a plasma accelerator is suitable, for example, for correcting the travel path or trajectory of satellites or rockets.

These applications require that accurately defined quantities of plasma having prescribed amounts of energy be ejected from the travel correction device. Such conditions are readily satisfied with the aid of the method and plasma accelerator according to the invention.

To those skilled in the art it will be obvious upon a study of this disclosure, that our invention is amenable to a variety of modifications and may be given embodiments other than particularly illustrated and described herein, without departing from the essential features of our invention and within the scope of the claims annexed hereto.

We claim:

1. Apparatus for accelerating plasmoids which comprises a plasma flow path, means for producing at least two cusp fields spaced along a plasma flow path, means for providing a variable magnetic field in proximity with the first cusp field in the direction of flow along said flow path, means for producing a substantially circular electrical field from said variable magnetic field for generating in a plasma traversing said flow path a circulating current flowing perpendicularly in the first cusp field so as to generate a force accelerating the plasma toward the second cusp field, means for collecting the plasma for a period of predetermined length at the second cusp field, means for heating the plasma in the second cusp field during said period so as to increase its electrical conductivity, and means for providing a variable magnetic field in proximity with the second cusp field and producing therefrom a substantially circular electrical field for generating in the collected plasma a circulating current flowing perpendicularly in the second cusp field so as to generate a force greater than said first-mentioned force in the second cusp field for accelerating the plasma in the direction of flow along the plasma flow path.

2. Apparatus according to claim 1 including means for limiting the collection period at the second cusp field to approximately 5 microseconds duration.

3. Apparatus according to claim 1 including means for varying the variable magnetic fields in proximity with the two cusp fields repeatedly so as to reflect the plasma back and forth between the fields, and means for collecting the plasma in the respective field for a period having a duration in the order of magnitude of several microseconds prior to reflecting the plasma to the other field so as to heat the plasma stepwise to higher temperatures.

4. Apparatus for electromagnetic control of plasmoids, comprising a tubular vessel of electrically insulating material defining a flow path traversible by a plasma, a plurality of magnetic field coils magnetically opposed to one another, said field coils coaxially surrounding said tubular vessel and mutually spaced axially from one another for producing in the respective spaces therebetween a magnetic field having a radial component, a plurality of induction coils coaxially surrounding said tubular vessels and located respectively in the spaces between adjacent field coils, electric energizing means connected to said field coils, and periodic electric energizing means con nected to said inductance coils for causing the periodic magnetic field of said inductance coils to produce in the vessel a circular electric field at the respective locations of said inductance coils so as to generate a circulating current in the radial magnetic field resulting in axially directed acceleration of the plasma, and means for delaying the energizing of at least the second induction coil located along the flow path of the plasma in said vessel so as to collect the plasma in a cusp field formed in said vessel at said second induction coil whereby the plasma is heatable to a higher temperature increasing its electrical conductivity and is axially directed, after the second induction coil is subsequently energized, at a greater acceleration than the acceleration produced at the preceding induction coil along the flow path.

5. Apparatus according to claim 4 including energizing delay means for at least two succeeding induction coils so as to alternately collect the plasma in respective cusp fields located at said induction coils whereby the plasma may be reflected back and forth between the cusp fields of the respective induction coils for heating the plasma stepwise to higher temperatures.

References Cited UNITED STATES PATENTS 2,997,436 8/1961 Little 313-161 JAMES W. LAWRENCE, Primary Examiner.

S. SOCHLOSSER, Assistant Examiner. 

1. APPARATUS FOR ACCELERATING PLASMOIDS WHICH COMPRISING A PLASMA FLOW PATH, MEANS FOR PRODUCING AT LEAST TWO CUSP FIELDS SPACED ALONG A PLASMA FLOW PATH, MEANS FOR PROVIDING A VARIABLE MAGNETIC FIELD IN PROXIMITY WITH THE FIRST CUSP FIELD IN THE DIRECTION OF FLOW ALONG SAID FLOW PATH, MEANS FOR PRODUCING A SUBSTANTIALLY CIRCULAR ELECTRICAL FIELD FROM SAID VARIABLE MAGNETIC FIELD FOR GENERATING IN A PLASMA TRAVERSING SAID FLOW PATH A CIRCULATING CURRENT FLOWING PERPENDICULARLY IN THE FIRST CUSP FIELD SO AS TO GENERATE A FORCE ACCELERATING THE PLASMA TOWARD THE SECOND CUSP FIELD, MEANS FOR COLLECTING THE PLASMA FOR A PERIOD OF PREDETERMINED LENGTH AT THE SECOND CUSP FIELD, MEANS FOR HEATING THE PLASMA IN THE SECOND CUSP FIELD DURING SAID PERIOD SO AS TO INCREASE ITS ELECTRICAL CONDUCTIVITY, AND MEANS FOR PROVIDING A VARIABLE MAGNETIC FIELD IN PROXIMITY WITH THE SECOND CUSP FIELD AND PRODUCING THEREFROM A SUBSTANTIALLY CIRCULAR ELECTRICAL FIELD FOR GENERATING IN THE COLLECTED PLASMA A CIRCULATING CURRENT FLOWING PERPENDICULARLY IN THE SECOND CUSP FIELD SO AS TO GENERATE A FORCE GENERATE THAN SAID FIRST-MENTIONED FORCE IN THE SECOND CUSP FIELD FOR ACCELERATING THE PLASMA IN THE DIRECTION OF FLOW ALONG THE PLASMA FLOW PATH. 